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Searching for Dark Matter, Underground

The Cryogenic Dark Matter Search (CDMS) is an experiment funded by the U.S. National Science Foundation (NSF) and the Department of Defense (DOE). It employs approximately 60 physicists across the U.S. and internationally, of which many are affiliated with U.C. Berkeley. CDMS looks for evidence of dark matter in our universe by attempting to measure signatures of putative particles called Weakly-Interacting Massive Particles (WIMPs). This is accomplished using cutting-edge detectors of silicon and germanium. WIMPs have no electric charge and easily pass through ordinary matter, making them extremely difficult to detect. WIMPs are believed to have been produced just moments after the Big Bang during the hot conditions of the early universe.

For decades, firm astronomical evidence from observations of stars and galaxies has indicated that most of the matter in the universe cannot be seen directly in telescopes. Instead, this matter must be observed indirectly through its gravitational pull on the objects that we can see. This is how the term "dark matter" was coined, and it is a compelling notion that dark matter is comprised of the WIMPs that CDMS seeks to find. In other words, it is possible that enough WIMPs were produced just after the Big Bang that they make up most of the matter in the universe today. The total mass of WIMPs would be the dominant source of the gravitational force that keeps stars orbiting around the centers of galaxies.

A CDMS Z-resolving Ionization and Phonon (ZIP) Detector.

So if WIMPs were produced in sufficient number in the early universe, then their properties are limited to a certain range. The detection mechansim is that occasionally individual WIMPs will occasionally collide with individual nuclei that make up one of the CDMS detectors. Since the recoiling nucleus is charged, it slows down and stops within the detector and its kinetic energy is dissipated in the detector and appears as both a heat signal and a charge signal. By comparing the two types of signals, recoils due to WIMPs can be distinguished from recoils due to most other types of particles.

Charged particles interact with the electronic structure of an atom, whereas WIMPs would interact with nuclei. Image courtesy Michael Attisha

Now, since WIMPs would only interact very rarely, CDMS must also shield the detectors from background sources of particles. For this reason, the final experiment is located in a mine in northern Minnesota to limit the influence of cosmic rays from space.

The mine head at the Soudan Underground Laboratory, Soudan, MN.

Furthermore, since the amount of heat deposited by these single particles is small, the calorimeters used on CDMS detectors must operate at a temperature of only 40 milliKelvin, i.e., 40/1000 of a degree above absolute zero. To operate rather massive detectors at such a low temperature requires dilution refrigeration. This refrigeration technique is based on heat exchange properties of a mixture of helium-3 and helium-4. Operationally, dilution refrigerators require high vacuum and an ongoing supply of liquid nitrogen and liquid helium.

The "Icebox" core of the CDMS experiment, with dilution refrigerator infrastructure.